1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave apparatus to be mounted using metal bumps by a flip chip bonding system and, also relates to the surface acoustic wave apparatus produced by such a method. In particular, the present invention relates to a method of manufacturing a surface acoustic wave apparatus in which at least one electrode for a surface acoustic wave element is formed by a lift-off method and, also relates to the surface acoustic wave apparatus.
2. Description of the Related Art
In recent years, in order to miniaturize surface acoustic wave apparatuses, the surface acoustic wave apparatuses assembled by a flip chip bonding system have been used widely. In this system, bumps made of Au or other material, are formed at electrode pads on a piezoelectric substrate constituting the surface acoustic wave apparatus, and the electrode pads and input and output electrode pads provided on the package or ground electrode pads are electrically connected via the bumps, and are mechanically joined at the same time.
When the aforementioned flip chip bonding system is used, the bumps not only electrically connect the surface acoustic wave apparatus and the package, but also mechanically fix the surface acoustic wave apparatus to the package. Therefore, it is required that the bumps have a high strength. In addition, the joining strength between the bumps and the electrode pads on the piezoelectric substrate must be high, and the adhesion between the electrode pads and the piezoelectric substrate must be high.
In order to increase the joining strength between the electrode pad and the bump, in general, a method, in which the thickness of the electrode pad is sufficiently increased, has been used. In order to increase the thickness of the electrode pad, a conventional method, in which a second electrode layer having a large film thickness is formed on a first electrode layer having a small film thickness, is known.
On the other hand, when the surface acoustic wave apparatus is formed, electrodes for the surface acoustic wave element, for example, an interdigital transducer, reflector, and wiring electrodes, and the aforementioned electrode pads are formed on the piezoelectric substrate. When the electrode pad includes the first and second electrode layers, in many cases, the electrodes for the surface acoustic wave element and the first electrode layer of the electrode pad are formed simultaneously. As the method for forming the electrode for surface acoustic wave element, (1) an etching method or (2) a lift-off method, has been used. In (1) the etching method, a conductive film primarily containing Al is formed over the entire surface of a substrate, and a desired resist pattern is formed by photolithography. Thereafter, the resulting metal film is processed by wet etching or dry etching, and then, the resist is removed. In (2) the lift-off method, the metal film portion adhered on the resist is removed together with the resist and, therefore, the electrode is formed from the remaining metal film portion.
In particular, regarding some surface acoustic wave filters for use in a 800 MHz band or in a 1 GHz to 2 GHz band, surface acoustic wave apparatuses are formed by the use of the aforementioned (2) lift-off method. An example of the method for manufacturing the aforementioned surface acoustic wave apparatus will now be described with reference to FIGS. 22 to 24.
As shown in FIG. 23A, a resist pattern 102 is formed on a piezoelectric substrate 101 by photolithography. A metal film 103 primarily containing Al is formed on the piezoelectric substrate 101 as shown in FIG. 23B. Subsequently, the resist pattern 102 is removed together with the metal film portion adhered thereon by a lift-off process. Thus, a first electrode layer 103a for constituting an electrode pad and an electrode for the surface acoustic wave element 103b are simultaneously formed on the piezoelectric substrate 101 as shown in FIG. 23C. Then, a resist pattern 104 is formed (FIG. 23D). A metal film 105 is formed as shown in FIG. 24A, and the resist pattern 104 is removed by performing the lift-off process again. Consequently, as shown in FIG. 24B, a second electrode layer 105a is formed on the first electrode layer 103a and, therefore, electrode pads 106 having a double-layer structure can be produced.
Next, as shown in FIG. 22, bumps 107 are joined onto the electrode pads 106. A surface acoustic wave apparatus 108 is joined with a package by a flip chip bonding system using the bumps 107.
Regarding the above-described prior art shown in FIGS. 22 to 24, in the case where the first electrode layer 103a of the electrode pad 106 was formed by the lift-off method, since the adhesion between the piezoelectric substrate 101 and the first electrode layer 103a was relatively weak due to the effects of the resist used for the lift-off, when the formation was performed using bumps 107 by a wire bump bonding method concurrently using ultrasonic waves and heat, sometimes, peeling occurred between the first electrode layer 103a and the piezoelectric substrate 101.
Furthermore, when the surface acoustic wave apparatus 108 was mounted on the package by the flip chip bonding system, and airtight sealing was performed by a covering member, sometimes cracks occurred in the piezoelectric substrate 101 in areas adjacent or near the electrode pads 106 due to the mechanical stress brought about by the residual stress. Therefore, the reliability, especially the reliability of the mechanical strength, of the surface acoustic wave apparatus was significantly degraded.